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Energy and the Environment
The economics of wastewater treatment decentralization: A techno-economic evaluation Manel Garrido-Baserba, Sergi Vinardell, Maria Molinos-Senante, Diego Rosso, and M. Poch Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01623 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 3, 2018
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The economics of wastewater treatment decentralization: A techno-
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economic evaluation
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Manel Garrido-Baserba1,2*, Sergi Vinardell3, María Molinos-Senante4,5, Diego Rosso1,2, Manel
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Poch3
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1 Department of Civil & Environmental Engineering, University of California, Irvine, CA 92697-2175, U.S.A. (Email:
[email protected])
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2
Water-Energy Nexus Center, University of California, Irvine, CA 92697-2175, U.S.A.
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3
LEQUiA, Institute of the Environment, University of Girona, E-17071, Girona, Spain
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4 Department of Hydraulic and Environmental Engineering, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile.
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5 Center for Sustainable Urban Development, CONICYT/FONDAP/15110020, Av. Vicuña Mackenna 4860, Santiago, Chile.
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*Corresponding author (T: +1-949-233-6446), E-mail:
[email protected])
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ABSTRACT
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The existing wastewater treatment infrastructure has not adequately established an efficient and
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sustainable use of energy, water, and nutrients. A proposed scheme based on source separation
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and water-efficient use is compared to the current wastewater management paradigm (one
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largely based on activated sludge) using techno-economic terms. This paper explores the
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economic viability of adopting more sustainable management alternatives and expands the
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understanding of the economics of decentralization and source-separation. The feasibility of
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three different potential types of source-separation (with different levels of decentralization) are
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compared to the conventional centralized activated sludge process by using recognized
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economic assessment methodologies together with widely accepted modeling tools. The
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alternatives were evaluated for two common scenarios: new developments and retrofit due to
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the aging of existing infrastructures. The results prove that source-separated alternatives can be
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competitive options despite existing drawbacks (only when countable incomes are included),
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while the hybrid approach resulted in the least cost-effective solution. A detailed techno-
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economic evaluation of the costs of decentralization provides insight into the current constraints
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concerning the paradigm shift and the cost of existing technologic inertia.
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KEY WORDS: Activated sludge; Decentralization; Water management; Source-separation;
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Nitrogen removal
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1.
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Economic and environmental sustainability are shifting the current paradigm in urban
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wastewater management
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worldwide water sanitation significantly after increased urbanization and industrialization by
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providing safe effluent from wastewater. But this process is now being recognized as lacking
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economic and environmental sustainability, especially with respect to the inefficient use of
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energy, (recycled) water, and nutrients
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and constructed on the basis of outdated views, requirements, conditions, and technologies of
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decades ago 8. Greenhouse gas emissions from the AS process itself (e.g., N2O) and from
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sources of energy production, lack of recovery of finite nutrients such as phosphorus (even
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given current depletion rates), continually rising energy costs, resilience limitations, and the
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need for cost-efficient technologies are among the forces that will drive cities to start building
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the next generation of urban wastewater solutions 3. Furthermore, the construction of new
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developments in cities will be a worldwide phenomenon in the coming decades 9. Similarly,
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existing and aging infrastructures in wastewater treatment facilities will soon need to be
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replaced.
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In the pursuit of sustainable wastewater management options, source separation and
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decentralization are slowly becoming realistic alternatives for these new and expected
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developments10,11. Both wastewater treatment alternatives lead to several advantages:
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Introduction
-
1–4
. The century-old activated sludge (AS) process improved
5–7
. Current wastewater infrastructures were designed
The organic carbon in a typical combined municipal wastewater represents a chemical
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energy content of approximately 1.9 kWh/m3
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potential to harvest this energy within the order of 0.6 to 0.9 kWh/m3 for the most
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concentrated BW stream in households
. While anaerobic digestion has the
, the current AS process on the other hand
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consumes about 0.3-0.7 kWh/ m of wastewater 15–17.
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12–14
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The current practice is to merge, dilute, and treat both gray wastewater (GW) and black
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wastewater (BW) streams, which hinders the feasibility of nutrient recovery. Highly
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concentrated BW streams can be treated separately to facilitate nutrient recovery
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Similarly, using a minimal amount of water yields concentrated wastewater flows which
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are more cost-effective for removing harmful micropollutants
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valuable constituents 22.
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-
20,21
18,19
.
or recovering
Recovering nitrogen could reduce the production of artificial fertilizers via the Haber-
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Bosch process, which fixes nitrogen from the air but uses up to 2% of the world’s
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energy 23 and represents 50% of the energy in European agriculture8.
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The energy demand to run aeration blowers in the aeration-based AS process accounts
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for more than 50-75% of the net power demand in wastewater treatment plants
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(WWTPs) needed to meet the mandated amount of dissolved oxygen 24–26.
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Source separation and decentralization could reduce the current increase in energy
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demand (and concurrent carbon footprint) caused by the implementation of new
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technologies that achieve higher effluent quality at the expense of higher energy
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demand
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avoiding energy-demanding AS processes and transport 28–30
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-
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by producing renewable energy in useful forms (heat, methane) and by
The current trend in clean decentralized energy (i.e., biogas, solar, wind) offers new
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possibilities of decentralized wastewater treatment, making new water reuse systems
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scalable, off-grid, and without the need for the transport of fossil fuels 31–36.
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-
Vacuum toilets, as a way of source separation, can reduce BW water consumption by 90% to 35 liters per person/day 10,37–40., and the overall consumption by about 25%41.
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Increase the ability of urban wastewater systems to adapt as a response to change 42 and enhance climate-resilient infrastructures 6,43–46.
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Despite new available knowledge, expertise, and technologies to develop more economically
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and
environmentally
sustainable 47–50
water
resource
management
alternatives,
practical
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implementation remains slow
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and hardly any resources were allocated to their development because they are still considered
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immature and risky by most wastewater professionals
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that developing alternative cost-efficient wastewater management systems is an issue of
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governance rather than technology
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consumers (>64%) express highly favorable views of new systems combining elements of
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source separation, local treatment, and reduced water use 54,55. A lack of evidence pertaining to
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the economic viability of these alternatives hinders their consideration as feasible and credible
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options. Therefore, we aimed to present a clear and simple approach to the economics of source
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separation and decentralization to provide sound information that can support the decision-
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making of (waste) water authorities.
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One of the first comparison studies to date stated that depending on the scenario, source
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separated systems are more cost effective than conventional systems
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instead estimated that the overall costs of source-separated approaches could be about twice the
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conventional system
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. Source separating technologies are considered “low-tech,”
50,52,53
10,50,51
. Similarly, some authors suggest
. Furthermore, recent studies have highlighted that
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. Other authors have
. However, none of these studies used standardized and recognized
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methods of cost assessment (i.e., licensed software), and their calculations were mainly based
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on their own experience and references. Including the potential incomes from source separation
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is also essential to evaluate decentralized alternatives 58.
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The aim of this paper is to expand the understanding of the economics of decentralization and
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source-separation by using standardized approaches for economic projections and evaluations in
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wastewater systems. The feasibility of three different potential types of source-separated
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systems were compared with the AS process using commercially available modeling software
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(i.e., CapdetWorks). The main novelty of this study is to provide a comprehensive comparison
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and assessment of wastewater treatment alternatives, including the following: 1) A methodology
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based on reliable cost-estimation software (i.e., CapdetWorks; Hydromantis, Inc.) and state-of-
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the-art literature for estimating construction and operation costs; 2) Consideration of only
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existing technologies (not requiring further innovation before their deployment) that have been
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accepted as feasible alternatives among wastewater experts; 3) Inclusion of the potential income
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produced by source-separated alternatives; 4) Implementation of the aforementioned analytical
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analysis in two of the most common scenarios in developing and developed societies: New
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wastewater treatment developments and the aging of existing infrastructures (retrofit),
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respectively. Therefore, a detailed and integrated economic analysis including the sewer system,
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existing and realistic alternative wastewater treatments, and resource efficiency is presented.
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2.
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2.1 Wastewater treatment alternatives
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Two main scenarios were considered for the economic assessment of the selected wastewater
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management alternatives: new developments, and the retrofit of an existing WWTP. Both
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scenarios are evaluated for a medium-sized population of 30,000 population equivalent (PE).
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The population size was selected to represent an average, intermediate city scenario in which
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activated sludge-related configurations would typically be the preferred option to implement.
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Moreover, the installation of anaerobic digesters (AD) is often discouraged in centralized
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communities smaller than 40,000 PE due to economic and technical reasons. Phosphorus re-
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solubilization during the hydrolysis step in AD drives highly concentrated phosphorus flows
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back to the main stream, leading to recirculation instead of actual P-removal, plus increased
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piping blockage by spontaneous struvite precipitation
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without anaerobic digestion may facilitate the comparison with source-separated alternatives
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(Anaerobic-based). Note that larger treatment plants would benefit from the use of side-stream
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AD (specifically by using co-digestion strategies) and operational savings should be included in
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the case of a techno-economic analysis.
Methodology
5959–62
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. The consideration of an AS
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Figure 1. Flow diagrams for three wastewater treatment alternatives. a) Fully centralized alternative using activated sludge; b) Hybrid alternative following a centralized approach for gray water and a decentralized approach for black water; c) Fully decentralized approach for both black and gray water streams. Alternatives B and C can have two differentiated treatments for the liquid effluent from the anaerobic processes unit (Figure 2).
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Figure 2. Flow diagram alternatives for the liquid effluent from the anaerobic processes unit. Option 1) Nitrogen recovery by a stripping-absorption system; Option 2) Nitrogen removal by the Oland process.
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For each scenario, three wastewater management alternatives or flow diagrams are applied: i)
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centralized (alternative A); ii) hybrid (alternative B1 and B2) and; iii) decentralized (alternative
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C) (Figures 1 and 2; See S12 for a detailed description in supporting information). The
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centralized alternative consisted of a typical AS process. Alternative B (hybrid) is characterized
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by a partially centralized scheme, while alternative C represents a fully decentralized scheme.
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For both source-separated alternatives (B and C), the treatment of nitrogen was evaluated
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considering two different technologies. One approach was based on the physical-chemical
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recovery of the nitrogen (i.e., stripping-absorption process, alternative B1), while the other was
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based on the biological removal of nitrogen (i.e. ,Oland/anammox process, alternative B2 and
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C). For the sake of simplicity, the results of the two nitrogen treatment alternatives were shown
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only for alternative B. For the decentralized alternative (C), only the output of the most cost-
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effective alternative, i.e., nitrogen removal by the Oland process, is presented in this paper. A
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detailed description of each alternative is provided as supplemental material.
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2.2 Influent composition
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Table 1 shows the typical values for an urban influent in a centralized WWTP, which
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traditionally combines BW and GW10. The wastewater composition shown in table 1 was
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selected as the influent for all the wastewater alternatives evaluated. In the source-separation
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cases, the two different streams were split accordingly, as shown in Table 1. Following current
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practices, BW is expected to be collected with vacuum toilets, which means a consumption of
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water per people equivalent (PE) of 5 L/PE/day 63. As for GW, its water consumption is about
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108 L/PE/day 10,64.
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Table 1. Typical pollutant concentrations for the three main influents (i.e., combined, gray and
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black water) 64–69. Combined
Source-separated streams
mg/L
mg/L (GW)
mg/L (BW)
TSS - Total Suspended Solids
410.8
175.9
8,360
COD - Chemical Oxygen Demand
701.3
472.2
10,560
BOD - Biological Oxygen Demand
248.6
175.9
3,560
TKN - Total Kjeldahl Nitrogen
86.1
2.3
2,500
NH4+_N - Ammonium-N
6.1
2.3
132.4
TP - Total Phosphorus
13.7
4.6
306.0
4.3
5.9
-
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N-NO3 - Nitrate
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Conventional toilets were used in the sewer combined alternative (alternative A, combining and
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not discerning between BW and GW), which means a consumption of about 40 L/p/day10.
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2.3 Sewer infrastructure
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For comparative and standardization purposes the sewer distribution of the new development
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(Scenario 1) and retrofit (Scenario 2) scenarios were adapted from Roefs et al. (2016). Each
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development consisted of a series of districts (See figure S4, supporting information) servicing
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1200 PE. Each district was distributed in neighborhoods of 50 households representing a total of
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120 PE. Each neighborhood had a surface area of 2.5 ha.
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In both the conventional and hybrid alternatives, districts were connected to a collection system
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that connected district to district and to the central WWTP. A backbone pipe was used as
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connection between neighborhoods within the district (Fig. S4). At the neighborhood level, both
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private and public sewers were taken into account. Private sewers were defined as the sewers
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from the house to the first Y-joint that makes connection with the water main
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description of sewer infrastructure is shown in the supplemental information.
56,70
. A detailed
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2.4 Model domain.
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Influent variability. This study assumes low variability in the influent concentrations for the
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source-separated options as the main uncertainty contributors are avoided: Industrial effluents
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(traditionally representing 15-30% of flow composition), storm water episodes, sewage
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characteristics (i.e., combined and separated), uncontrolled infiltrations or additions, public
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spaces effluents (i.e., swimming pools, malls), etc. It is assumed that each person would produce
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a similar waste water composition (Table1) from the daily theoretical 1.2-1.5l of urine and feces
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further studies should be carried out to determine the exact impact of non-average
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concentrations.
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On the other hand, hybrid and centralized alternatives will inevitably be sensitive to load
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fluctuations that cannot be predicted with the present methodology. The reduction of uncertainty
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could be reduced further by studies incorporating wastewater dynamics and site-specific
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fluctuation.
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Sewer System. This study was based on a density of about 20 household/hectare, which is a
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common value for European residential areas with free standing or double houses
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studies that explore the variability depending on other common population densities would be
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required, as there can be lower densities (i.e., US and Canada) or higher densities (i.e., highly
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populated cities). The model used in this study was developed by Maurer et al. (2013), and may
. Nevertheless, the authors acknowledge the potential influence of influent variability and
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. Further
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yield significantly different results, especially if some pipe layouts needed to be arranged
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differently. Furthermore, the impact of assuming different distances to the central plant
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(currently 3km) may result in close to negligible results for distances shorter than 30-40km
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(following the aforementioned methodology). The methodology considers straight long-distance
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pipes (Main column or artery, HDPE) with significantly lower costs (
